Monday, March 17, 2014

I hear a vague distant sound sometimes, like a locomotive engine that never leaves. It came up earlier in the year, but beyond suggesting it sounded like it was coming from the SE from my location, didn't do anything else. Recently it came up again, and evidently for some people, is intolerable. These people live east of the Metra Electric tracks in various high-rises and other multistory buildings between 51st and 56th St (specifically, people in the Windemere, 1700 E. 56th, 5490 South Shore, 5479 S. Hyde Park, and 51st and East End have mentioned it). People on the west side of 1700 E. 56th higher up (like 28th floor and above) seem to have it the worst--and they claim their management have turned off every system in their building to prove it wasn't local to them. I originally thought it was related to the opening of Earl Shapiro Hall on Stony Island, to the SE of me.

I went wandering around a few weekends ago on a slightly breezy and roughly about freezing 0C evening with an ipad running a tuner app that offered a fairly nice spectrum analyzer and I could see a prominent tone at about 85Hz that came and went, and varied in intensity. It turned on and off with a period of several minutes. I saw it best in quiet places. I saw it underneath the Metra tracks at 57th. I did not see it get any louder when I pressed the tablet against the UC maintenance building on Stony Island between 56th and 57th. I saw it at 54th and Cornell. I did not see it at 53rd and South Shore. I saw it in the alley between Everett and Hyde Park Blvd between 53rd and 55th. I see it in front of the home at 5522 S Hyde Park Blvd. I see it quite strongly in the alley between Cornell and Hyde Park Blvd between 55th and 56th, strongest at the back of 5528 S Hyde Park Blvd. It is quite strong there and next door at 5540 S Hyde Park Blvd, the Poinsettia Apts and Broadview Hall, respectively. Given that, I wondered if my aural direction of SE is real, or some reflection from Vista Homes.

The iPad software is limited to being sensitive to approximately 40Hz and higher, with a realistic limit of about 60Hz.

Things it is not: It is not related to the power grid, which mostly produces noise at 120Hz, with 60Hz appearing occasionally, plus all harmonics. I am very familiar with that noise.
It is not a moving train. When a freight train goes by, the locomotive produces several harmonics in the sound area while it passes, but they have a Doppler component. The freight component offers deep rumbling at 9 to 11Hz.
Metra trains produce lots of frequencies between 100-200Hz.

I have a PC laptop at home with a microphone running Spectrum Laboratory and Spectran, which I can vary the FFT size, sampling rate, and other variables which allow me to get much lower in frequency. The practical limit is 10Hz which seems to be the microphone's limit. That software is not calibrated, and I suspect it is off by 10Hz, since I see a discrepancy between the iPad, the PC, and what I expect to see.

I have seen it at home in the early evening, evening, at night, in the morning. I have not been at home during the day very much, but I suspect it is there then as well.

I embed an image of the monitoring software taken last night, 3/16/2014, between 10:30PM and 11PM below. Time is horizontal, frequency goes up vertical. Anything that is strongly vertical is likely me moving around, very local, moving the laptop, etc. Like at 22:37 I think I moved from my midlevel upstairs. You can see the "110Hz" horizontal line, which if my PC is off by 10Hz, could be the ubiquitous 120Hz AC hum omnipresent.

The thing I call the 85Hz noise you can see is recorded here between 70 and 80Hz. And it has some variance in frequency, always in the same pattern, always changing in a discrete manner.
Another fairly signifcant tone is visible starting at about 22:31, in between 30 and 40Hz. It gets quieter when I go upstairs at 22:37. You see it changes discretely as well, but in a different way, than the "85Hz" tone. A discrete change like this is usually the sign of a controlled system.

The wispy, broadband thing at 22:52? I believe that was a jet plane. I don't recall that specific one, but that's what they look like.

A garbage truck going down the street looks like this:
. The horizontal ticks are one minute apart here and with a different sampling rate and such the 120Hz appears to show up at 120Hz.

There's more on the flickr stream, if you take a look. I think this one https://www.flickr.com/photos/dwarmstr/13210307253/ is earlier yesterday evening and you can see more of the discrete hum changes, plus non-discrete changes, etc. It's hard to pin down.

I also was running spectran on some screenshots, showing lower frequency noises. You can see prominent tones at 46-47Hz and 29Hz:
.

One question I do have is wondering if everyone is complaining or hearing the same sound. The different aspects of the "85Hz" tone vs the stuff at 30-40Hz is suggestive of two different systems, and yet they are both prominent. One could be the window rattler and the other could be the one you actually "hear". This is all speculation on my part. There's a lot of further investigating (and speculating!) to be done here, but it will take time. In an ideal world one would be able to visit the facilities of various buildings to see if the systems are there or not. Or have more time to measure the intensity in a number of places to get a better map. I thought I had found the source at 5528 because there is a metal chimney pipe the length of the building that seemed to be at the loudest area. In retrospect, there is a large pipe at Broadview in the rough area of the loudest ground signal, but it's elevated and away from easy access.

What is the source? Is it some fundamental of a system motor, it is a resonance of a exhaust pipe? Why does it change?

Atmospheric effects can bend sound down and return loud noises from the ground back to distant areas because the sound is gently bent from the change in the speed of sound with temperature. Are we hearing something more distant like a train switching station from somewhere else?